All-solid lithium secondary battery, and deterioration determination method of all-solid lithium secondary battery

ABSTRACT

An all-solid lithium secondary battery includes a positive electrode active material layer, a metallic lithium absorption layer, a solid electrolyte layer, and a negative electrode active material layer in this order. The solid electrolyte layer is in contact with the negative electrode active material layer. The metallic lithium absorption layer contains a metallic lithium reactive substance. The metallic lithium reactive substance reacts with metallic lithium to generate an electron conductor which is stable under charging and discharging conditions of the all-solid lithium secondary battery.

INCORPORATION BY REFERENCE

The disclosure of Japanese Patent Application No. 2018-215045 filed onNov. 15, 2018 including the specification, drawings and abstract isincorporated herein by reference in its entirety.

BACKGROUND 1. Technical Field

The present disclosure relates to an all-solid lithium secondary batteryand a deterioration determination method of an all-solid lithiumsecondary battery.

2. Description of Related Art

Lithium secondary batteries have features that they have a higher energydensity than other secondary batteries and can be operated at a highvoltage. Thus, lithium secondary batteries are used for informationdevices such as mobile phones as secondary batteries that can be easilyreduced in size and weight, and in recent years, the demand for largepower sources for electric vehicles, hybrid vehicles and the like hasincreased.

Regarding lithium secondary batteries, it is known that, depending onthe configuration and a manner of use of the battery, metallic lithiumdendrites grow due to repeated charging and discharging and the like andreach a positive electrode active material layer from a negativeelectrode active material layer, which results in internal shortcircuiting.

Examples of techniques for restricting such internal short circuitinginclude WO 2015/182615, Japanese Unexamined Patent ApplicationPublication No. 2009-301959 (JP 2009-301959 A), and Japanese UnexaminedPatent Application Publication No. 2009-211910 (JP 2009-211910 A).

WO 2015/182615 discloses a secondary battery including a positiveelectrode active material layer, a negative electrode active materiallayer made of an alkali metal, a separator which is made of atetrafluoroethylene (TFE) polymer or copolymer that reacts with analkali metal dendrite, and in which a hydrophilization treatment isperformed in a proportion of 10% or more and 80% or less, and a layerwhich is positioned between the separator and the negative electrodeactive material layer and does not react with an alkali metal dendrite.

In addition, JP 2009-301959 A discloses an all-solid lithium secondarybattery having a surface vapor deposition film in which a solidelectrolyte is deposited between a negative electrode active materiallayer and a solid electrolyte layer, and/or between a negative electrodeactive material layer and a solid electrolyte layer by a gas phasemethod.

In addition, JP 2009-211910 A discloses an all-solid lithium secondarybattery in which there is a liquid substance that reacts with metalliclithium to generate an electronic insulator in a solid electrolyte layerof a powder molded article obtained by molding a solid electrolytepowder.

Here, in lithium secondary batteries, for purposes other thanrestricting internal short circuiting, for example, reducing theinternal resistance, improving the ionic conductivity, or improving theenergy density, a plurality of solid electrolyte layers and the like maybe disposed between a positive electrode active material layer and anegative electrode active material layer. Examples thereof includeJapanese Unexamined Patent Application Publication No. 2014-238925 (JP2014-238925 A) and Japanese Unexamined Patent Application PublicationNo. 2009-259696 (JP 2009-259696 A).

JP 2014-238925 A discloses a lithium secondary battery in which apolymer solid electrolyte layer and an inorganic solid electrolyte layerare disposed between a positive electrode active material layer and anegative electrode active material layer.

In addition, JP 2009-259696 A discloses a lithium secondary battery inwhich an interface layer is disposed between a negative electrode activematerial layer and a solid electrolyte layer, and also a lithiumsecondary battery in which a buffer layer is disposed between a positiveelectrode active material layer and a solid electrolyte layer.

SUMMARY

As described above, as disclosed in, for example, WO 2015/182615, JP2009-301959 A, and JP 2009-211910 A, lithium secondary batteries thatrestrict internal short circuiting by restricting growth of metalliclithium dendrites are known.

However, in these lithium secondary batteries, when restriction ofgrowth of metallic lithium dendrites is not sufficient, if charging anddischarging are repeated, metallic lithium dendrites will eventuallyreach the positive electrode active material layer, and internal shortcircuiting can be caused.

In such a case, it is difficult to detect deterioration of a lithiumsecondary battery due to growth of metallic lithium dendrites until themetallic lithium dendrites reach the positive electrode active materiallayer and internal short circuiting occurs.

The present disclosure provides an all-solid lithium secondary batterythat can restrict internal short circuiting and detect whether theall-solid lithium secondary battery has deteriorated before internalshort circuiting occurs, and a method of detecting whether an all-solidlithium secondary battery has deteriorated.

A first aspect of the present disclosure relates to an all-solid lithiumsecondary battery including a positive electrode active material layer,a metallic lithium absorption layer containing a metallic lithiumreactive substance that reacts with metallic lithium to generate anelectron conductor which is stable under battery charging anddischarging conditions, a first solid electrolyte layer, and a negativeelectrode active material layer that is in contact with the first solidelectrolyte layer. The positive electrode active material layer, themetallic lithium absorption layer, the first solid electrolyte layer,and the negative electrode active material layer are disposed in thisorder.

The all-solid lithium secondary battery may further include a secondsolid electrolyte layer between the positive electrode active materiallayer and the metallic lithium absorption layer.

The metallic lithium reactive substance may have lithium ionconductivity.

The metallic lithium reactive substance may be a solid electrolytecontaining Li, P, S, and M. M may be Ge, Si, Sn, or a combinationthereof.

The metallic lithium reactive substance may be aLi_(3.25)Ge_(0.25)P_(0.75)S₄, Li₁₀GeP₂S₁₂, Li₁₀SnP₂S₁₂, Li₁₁Si₂PS₁₂, orLi₄GeS₄—Li₃PS₄ glass ceramic, a Li—Si—P—S—Cl solid electrolyte having anLGPS type structure, or a combination thereof.

The negative electrode active material layer may contain metalliclithium.

A second aspect of the present disclosure relates to a method ofdetermining a deterioration state of an all-solid lithium secondarybattery, including a first process of charging and discharging theall-solid lithium secondary battery; a second process of measuring acharging capacity and a discharging capacity of the all-solid lithiumsecondary battery during the charging and discharging; and a thirdprocess of determining a deterioration state of the all-solid lithiumsecondary battery from the relationship between the discharging capacityand the charging capacity.

In the third process, when the difference between the dischargingcapacity and the charging capacity is equal to or greater than a firstthreshold value or when a proportion of the charging capacity withrespect to the discharging capacity is equal to or lower than a secondthreshold value, it may be determined that the all-solid lithiumsecondary battery has deteriorated.

According to the present disclosure, it is possible to provide anall-solid lithium secondary battery that can restrict internal shortcircuiting and detect whether the all-solid lithium secondary batteryhas deteriorated before internal short circuiting occurs, and a methodof detecting whether an all-solid lithium secondary battery hasdeteriorated.

BRIEF DESCRIPTION OF THE DRAWINGS

Features, advantages, and technical and industrial significance ofexemplary embodiments of the disclosure will be described below withreference to the accompanying drawings, in which like numerals denotelike elements, and wherein:

FIG. 1A is a schematic view of an all-solid lithium secondary batteryhaving no mechanism for restricting dendrite growth;

FIG. 1B is a schematic view showing a dendrite growth state when theall-solid lithium secondary battery shown in FIG. 1A is charged anddischarged;

FIG. 1C is a schematic view showing a dendrite growth state when theall-solid lithium secondary battery shown in FIG. 1A is charged anddischarged;

FIG. 2A is a schematic view of an all-solid lithium secondary batteryhaving a shut layer;

FIG. 2B is a schematic view showing a dendrite growth state when theall-solid lithium secondary battery shown in FIG. 2A is charged anddischarged;

FIG. 2C is a schematic view showing a dendrite growth state when theall-solid lithium secondary battery shown in FIG. 2A is charged anddischarged;

FIG. 3A is a schematic view of an all-solid lithium secondary batteryaccording to an embodiment of the present disclosure;

FIG. 3B is a schematic view showing a dendrite growth state when theall-solid lithium secondary battery shown in FIG. 3A is charged anddischarged;

FIG. 3C is a schematic view showing a dendrite growth state when theall-solid lithium secondary battery shown in FIG. 3A is charged anddischarged;

FIG. 3D is a schematic view of an all-solid lithium secondary batteryaccording to a modification of the embodiment of the present disclosure;

FIG. 4 is a graph showing the charging and discharging capacities whenan all-solid lithium secondary battery of Example 1 is charged anddischarged;

FIG. 5 is a graph showing the charging and discharging capacities whenan all-solid lithium secondary battery of Example 2 is charged anddischarged;

FIG. 6 is a graph showing the charging and discharging capacities whenan all-solid lithium secondary battery of Example 3 is charged anddischarged;

FIG. 7 is a graph showing the charging and discharging capacities whenan all-solid lithium secondary battery of Comparative Example 1 ischarged and discharged;

FIG. 8 is a graph showing the charging and discharging capacities whenan all-solid lithium secondary battery of Comparative Example 2 ischarged and discharged;

FIG. 9 is a graph showing the charging and discharging capacities whenan all-solid lithium secondary battery of Comparative Example 3 ischarged and discharged; and

FIG. 10 is a graph showing the charging and discharging capacities whenan all-solid lithium secondary battery of Comparative Example 4 ischarged and discharged.

DETAILED DESCRIPTION OF EMBODIMENTS

Embodiments of the present disclosure will be described below in detail.Here, the present disclosure is not limited to the followingembodiments, and various modifications can be made within the scope ofthe gist of the disclosure.

«All-Solid Lithium Secondary Battery»

An all-solid lithium secondary battery of the present disclosure has apositive electrode active material layer, a metallic lithium absorptionlayer, a solid electrolyte layer, and a negative electrode activematerial layer in this order. Here, the solid electrolyte layer is incontact with the negative electrode active material layer. In addition,the metallic lithium absorption layer contains a metallic lithiumreactive substance. The metallic lithium reactive substance reacts withmetallic lithium to generate a stable electron conductor under batterycharging and discharging conditions.

The lithium secondary battery of the present disclosure can have, forexample, a structure having a positive electrode current collectorlayer, a positive electrode active material layer, a metallic lithiumabsorption layer, a solid electrolyte layer, a negative electrode activematerial layer, and a negative electrode current collector layer in thisorder. In addition, the lithium secondary battery of the presentdisclosure may have a structure in which a solid electrolyte layer isadditionally provided between a positive electrode active material layerand a metallic lithium absorption layer.

Although not limited by this principle, the principle under which theall-solid lithium secondary battery of the present disclosure canrestrict internal short circuiting and it is possible to detect whetheran all-solid lithium secondary battery has deteriorated before internalshort circuiting occurs is as follows.

First, a dendrite growth state in an all-solid lithium secondary batteryhaving no mechanism for restricting dendrite growth will be described.

FIG. 1A is a schematic view of an all-solid lithium secondary battery 10a having no mechanism for restricting dendrite growth. In FIG. 1A, theall-solid lithium secondary battery 10 a has a positive electrodecurrent collector layer 1, a positive electrode active material layer 2,a solid electrolyte layer 3, a negative electrode active material layer4, and a negative electrode current collector layer 5 in this order.

In addition, FIGS. 1B and 1C are schematic views showing growth statesof a dendrite 20 when the all-solid lithium secondary battery 10 a shownin FIG. 1A is charged and discharged. Here, FIG. 1B shows a state inwhich the all-solid lithium secondary battery 10 a in which the dendrite20 have grown to some extent is charged. In addition, FIG. 1C shows astate in which the all-solid lithium secondary battery 10 a in the statein FIG. 1B is discharged.

As shown in FIG. 1B, in the all-solid lithium secondary battery 10 a,metallic lithium dendrite 20 can grow from the side of the negativeelectrode active material layer 4 according to charging.

As shown in FIG. 1C, the grown dendrite 20 decomposes into lithium ionsand disappears or shrinks during discharging, and a gap 30 remains in apart in which the dendrite 20 was formed. Therefore, there is nosignificant change in the charging and discharging capacities, andapparently, normal charging and discharging occur. Therefore, it isdifficult to detect a growth state of the dendrite 20 until the dendrite20 grows and reaches the positive electrode active material layer 2 andinternal short circuiting occurs.

Next, a dendrite growth state in an all-solid lithium secondary batteryhaving a shut layer, that is, a layer containing a substance that reactswith a dendrite to generate an electronic insulator will be described.Here, examples of a substance that reacts with a dendrite to generate anelectronic insulator include tetrafluoroethylene (TFE) described in WO2015/182615.

FIG. 2A is a schematic view of an all-solid lithium secondary battery 10b having a shut layer 6. In FIG. 2A, the all-solid lithium secondarybattery 10 b has the positive electrode current collector layer 1, thepositive electrode active material layer 2, the shut layer 6, the solidelectrolyte layer 3, the negative electrode active material layer 4, andthe negative electrode current collector layer 5 in this order.

In addition, FIGS. 2B and 2C are schematic views showing growth statesof the dendrite 20 when the all-solid lithium secondary battery 10 bhaving the shut layer 6 is charged and discharged. Here, FIG. 2B shows astate in which the all-solid lithium secondary battery 10 b in which themetallic lithium dendrite 20 has grown to the shut layer 6 is charged.In addition, FIG. 2C shows a state in which the all-solid lithiumsecondary battery 10 b in the state in FIG. 2B is discharged.

As shown in FIG. 2B, when the all-solid lithium secondary battery 10 bis repeatedly charged and discharged, the dendrite 20 gradually growsfrom the side of the negative electrode active material layer 4 andreaches the shut layer 6. The dendrite 20 that has reached the shutlayer 6 reacts with a substance that reacts with a dendrite to generatean electronic insulator in the shut layer 6 to form an electronicinsulator 25. Thereby, additional growth of the dendrite 20 toward thepositive electrode active material layer 2 is restricted.

In addition, as shown in FIG. 2C, the grown dendrite 20 decomposes intolithium ions and disappears or shrinks during discharging, and the gap30 remains in a part in which the dendrite 20 is formed. Since theelectronic insulator 25 does not decompose into lithium ions duringdischarging, an irreversible capacity is generated in the all-solidlithium secondary battery 10 b. However, the electronic insulator 25 isonly formed at an interface between the solid electrolyte layer 3 andthe shut layer 6 in the dendrite 20, and an amount thereof is verysmall. Therefore, there is no significant change in the charging anddischarging capacities, and apparently, normal charging and dischargingoccur. Therefore, it is difficult to detect a growth state of thedendrite 20. In particular, when restriction of the growth of thedendrite 20 is not sufficient, it is difficult to detect a growth stateof the dendrite 20 until the dendrite 20 grows and reaches a positiveelectrode active material layer, and internal short circuiting occurs.

The all-solid lithium secondary battery of the present disclosure has apositive electrode active material layer, a metallic lithium absorptionlayer, a solid electrolyte layer, and a negative electrode activematerial layer in this order, and it is possible to detect deteriorationof the lithium secondary battery due to the growth of metallic lithiumdendrites before internal short circuiting occurs. Hereinafter, theprinciple will be described with reference to FIG. 3A and FIG. 3B, butthe all-solid lithium secondary battery of the present disclosure is notlimited to the configuration shown in FIG. 3A and FIG. 3B.

FIG. 3A is a schematic view of an all-solid lithium secondary battery 10c according to an embodiment of the present disclosure. In FIG. 3A, theall-solid lithium secondary battery 10 c has the positive electrodecurrent collector layer 1, the positive electrode active material layer2, a metallic lithium absorption layer 7, the solid electrolyte layer 3,the negative electrode active material layer 4, and the negativeelectrode current collector layer 5 in this order.

In addition, FIG. 3B is a schematic view showing a dendrite growth statewhen the all-solid lithium secondary battery 10 c shown in FIG. 3A ischarged and discharged. Here, FIG. 3B shows a state in which theall-solid lithium secondary battery in which a dendrite has grown to ametallic lithium absorption layer is charged. In addition, FIG. 3C showsa state in which the all-solid lithium secondary battery in the state inFIG. 3B is discharged.

As shown in FIG. 3B, when the all-solid lithium secondary battery 10 cis repeatedly charged and discharged, the dendrite 20 gradually growsfrom the side of the negative electrode active material layer 4 andreaches the metallic lithium absorption layer 7. The dendrite 20 thathas reached the metallic lithium absorption layer 7 reacts with ametallic lithium reactive substance contained in the metallic lithiumabsorption layer 7 to generate a stable electron conductor 27 underbattery charging and discharging conditions. Therefore, it is possibleto restrict additional growth of the metallic lithium dendrite 20 towardthe positive electrode active material layer.

The electron conductor 27 receives electrons from the dendrite 20 thatextends from the negative electrode active material layer 4 duringcharging. Therefore, the reaction between metallic lithium and themetallic lithium reactive substance further proceeds at an interfacebetween the electron conductor 27 and the metallic lithium reactivesubstance.

In addition, as shown in FIG. 3C, since the electron conductor 27 isstable under battery charging and discharging conditions, no lithiumions are generated when the battery is discharged, and an irreversiblecapacity is generated. Therefore, when the dendrite 20 grows and reachesthe metallic lithium absorption layer 7, the discharging capacity of theall-solid lithium secondary battery is significantly lower than thecharging capacity.

Therefore, in the all-solid lithium secondary battery of the presentdisclosure, when the charging and discharging capacities are measured,it is possible to detect the fact that the dendrite has grown andreached the metallic lithium absorption layer.

<Metallic Lithium Absorption Layer>

The metallic lithium absorption layer contains a metallic lithiumreactive substance.

(Metallic Lithium Reactive Substance)

The metallic lithium reactive substance is a substance that reacts withmetallic lithium to generate an electron conductor which is stable underbattery charging and discharging conditions. When the metallic lithiumreactive substance reacts with metallic lithium, it may generate, forexample, a substance having no ion conductivity and/or electronconductivity in addition to the electron conductor.

The metallic lithium reactive substance is preferably a substance havinglithium ion conductivity, for example, a solid electrolyte. Therefore,the metallic lithium reactive substance can be referred to as a solidelectrolyte which has a significantly greater tendency to generate theabove stable electron conductor than a solid electrolyte used in thesolid electrolyte layer.

When the metallic lithium reactive substance contained in the metalliclithium absorption layer has lithium ion conductivity, it is possible torestrict an increase in internal resistance of the all-solid lithiumsecondary battery due to the disposition of the metallic lithiumabsorption layer.

Specifically, the metallic lithium reactive substance may be a solidelectrolyte containing Li, P, S, and M as components. Here, M is Ge, Si,Sn, or a combination thereof.

Examples of such a composition include Li_(3.25)Ge_(0.25)P_(0.75)S₄,Li₁₀GeP₂S₁₂, Li₁₀SnP₂S₁₂, Li₁₁Si₂PS₁₂, and Li₄GeS₄—Li₃PS₄ glassceramics, a Li—Si—P—S—Cl solid electrolyte having an LGPS (Li₁₀GeP₂S₁₂)type structure, and combinations thereof.

Here, for example, when the metallic lithium reactive substance isLi₁₀GeP₂S₁₂, the reaction between the metallic lithium reactivesubstance and metallic lithium is expressed by the following Formulae(a) to (c).

Li₁₀GeP₂S₁₂+4Li⁺+4e ⁻→Ge+4Li₂S+2Li₃PS₄  (a)

Ge+yLi⁺ ye ⁻→Li_(y)Ge  (b)

Li₃PS₄+(5+x)Li⁺+(5+x)e ⁻→Li_(x)P+4Li₂S  (c)

Ge and Li₃PS₄ generated in Formula (a) each react with metallic lithiumin Formulae (b) and (c). More specifically, in Formula (b), Ge reactswith metallic lithium to generate Li_(y)Ge, and in Formula (c), Li₃PS₄reacts with metallic lithium to generate Li_(x)P and 4Li₂S.

Here, the products Li_(X)P and 4Li₂S in Formula (c) both have neitherelectron conductivity nor ion conductivity. However, the productLi_(y)Ge in Formula (b) is an electron conductor and is stable underbattery charging and discharging conditions.

<Solid Electrolyte Layer>

One solid electrolyte layer of the lithium secondary battery of thepresent disclosure is in contact with the negative electrode activematerial layer.

As shown in FIG. 3D, the lithium secondary battery of the presentdisclosure may have additionally another solid electrolyte layer 8 whichcontacts the positive electrode active material layer 2 and is providedbetween the positive electrode active material layer 2 and the metalliclithium absorption layer 7.

The solid electrolyte layer of the lithium secondary battery of thepresent disclosure can contain a solid electrolyte and an optionalbinder. Regarding the solid electrolyte, any material which has lowreactivity with metallic lithium and can be used as a solid electrolyteof the all-solid battery can be used. For example, the solid electrolytemay be a crystalline or amorphous sulfide solid electrolyte or acrystalline or amorphous oxide solid electrolyte, but the presentdisclosure is not limited thereto. In addition, the solid electrolytemay be a powder or a sintered product may be used.

Examples of sulfide solid electrolytes include a sulfide amorphous solidelectrolyte, a sulfide crystalline solid electrolyte, and an argyroditesolid electrolyte, but the present disclosure is not limited thereto.Specific examples of sulfide solid electrolytes include Li₂S—P₂S₅ types(Li₇P₃S₁₁, Li₃PS₄, Li₈P₂S₉, and the like), Li₂S—SiS₂, LiI—Li₂S—SiS₂,LiI—Li₂S—P₂S₅, LiI—LiBr—Li₂S—P₂S₅, LiI—Li₂S—P₂O₅, LiI—Li₃PO₄—P₂S₅,Li_(7−x)PS_(6−x)Cl_(x) and the like; and combinations thereof, but thepresent disclosure is not limited thereto.

Examples of oxide solid electrolytes include Li₇La₃Zr₂O₁₂,Li_(7−x)La₃Zr_(1−x)Nb_(x)O₁₂, Li_(7−3x)La₃Zr₂Al_(x)O₁₂,Li_(3x)La_(2/3−x)TiO₃, Li_(1+x)Al_(x)Ti_(2−x)(PO₄)₃,Li_(1+x)Al_(x)Ge_(2−x)(PO₄)₃, Li₃PO₄, and Li_(3+x)PO_(4−x)N_(x)(LiPON),but the present disclosure is not limited thereto.

The solid electrolyte may be glass or crystallized glass (glassceramic). In addition, the solid electrolyte layer may contain a binderand the like as necessary in addition to the above solid electrolytes.Specific examples are the same as “binders” listed in the following“positive electrode active material layer.”

<Positive Electrode Active Material Layer>

The positive electrode active material layer includes at least apositive electrode active material, and preferably further includes asolid electrolyte mentioned in the above solid electrolyte layer. Inaddition, according to intended uses, intended purposes, and the like,for example, additives used for the positive electrode active materiallayer of the all-solid battery such as a conductive aid and a binder canbe included.

The material of the positive electrode active material is notparticularly limited. For example, the positive electrode activematerial may be lithium cobalt oxide (LiCoO₂), lithium nickelate(LiNiO₂), lithium manganite (LiMn₂O₄), LiCo_(1/3)Ni_(1/3)Mn_(1/3)O₂, aheteroelement-substituted Li—Mn spinel having a composition representedby Li_(1+x)Mn_(2−x−y)M_(y)O₄(M is at least one metal element selectedfrom among Al, Mg, Co, Fe, Ni, and Zn), or the like, but the presentdisclosure is not limited thereto.

The conductive aid is not particularly limited. For example, theconductive aid may be a carbon material such as a vapor grown carbonfiber (VGCF) and a carbon nanofiber, a metal material, or the like, butthe present disclosure is not limited thereto.

The binder is not particularly limited. For example, the binder may be amaterial such as polyvinylidene fluoride (PVdF), carboxymethyl cellulose(CMC), butadiene rubber (BR) or styrene butadiene rubber (SBR), or acombination thereof, but the present disclosure is not limited thereto.

«Negative Electrode Active Material Layer»

The negative electrode active material layer includes at least anegative electrode active material, and preferably further includes theabove solid electrolytes. In addition, according to intended uses,intended purposes, and the like, for example, additives used for thenegative electrode active material layer of the lithium ion secondarybattery such as the above conductive aid and binder can be included.

(Negative Electrode Active Material)

The material of the negative electrode active material is notparticularly limited, and may be metallic lithium and may be a materialthat can occlude and release metal ions such as lithium ions. Regardingthe material that can occlude and release metal ions such as lithiumions, for example, the negative electrode active material may be analloy-based negative electrode active material, a carbon material, orthe like, but the present disclosure is not limited thereto.

The alloy-based negative electrode active material is not particularlylimited, and examples thereof include a Si alloy-based negativeelectrode active material and a Sn alloy-based negative electrode activematerial. Examples of Si alloy-based negative electrode active materialsinclude silicon, silicon oxide, silicon carbide, silicon nitride, andsolid solutions thereof. In addition, the Si alloy-based negativeelectrode active material can contain elements other than silicon, forexample, Fe, Co, Sb, Bi, Pb, Ni, Cu, Zn, Ge, In, Sn, and Ti. Examples ofSn alloy-based negative electrode active materials include tin, tinoxide, tin nitride, and solid solutions thereof. In addition, the Snalloy-based negative electrode active material can contain elementsother than tin, for example, Fe, Co, Sb, Bi, Pb, Ni, Cu, Zn, Ge, In, Ti,and Si. Among these, the Si alloy-based negative electrode activematerial is preferable.

The carbon material is not particularly limited, and examples thereofinclude hard carbon, soft carbon, and graphite.

<Current Collector Layer>

The lithium secondary battery of the present disclosure can have, forexample, a structure having a positive electrode current collectorlayer, a positive electrode active material layer, a metallic lithiumabsorption layer, a solid electrolyte layer, a negative electrode activematerial layer, and a negative electrode current collector layer in thisorder.

(Positive Electrode Current Collector Layer)

The material used for the positive electrode current collector layer isnot particularly limited, and those that can be used for the all-solidbattery may be appropriately used. For example, the material used forthe positive electrode current collector layer may be SUS, aluminum,copper, nickel, iron, titanium, carbon, or the like, but the presentdisclosure is not limited thereto.

The shape of the positive electrode current collector layer is notparticularly limited, and examples thereof include a foil shape, a plateshape, and a mesh shape. Among these, a foil shape is preferable.

(Negative Electrode Current Collector Layer)

The material used for the negative electrode current collector layer isnot particularly limited, and those that can be used for the all-solidbattery may be appropriately used. For example, the material used forthe negative electrode current collector layer may be SUS, aluminum,copper, nickel, iron, titanium, carbon, or the like, but the presentdisclosure is not limited thereto.

The shape of the negative electrode current collector layer is notparticularly limited, and examples thereof include a foil shape, a plateshape, and a mesh shape. Among these, a foil shape is preferable.

«Deterioration Determination Method»

A deterioration determination method of the present disclosure includesthe following processes (A) to (C):

(A) charging and discharging an all-solid lithium secondary battery ofthe present disclosure,(B) measuring a charging capacity and a discharging capacity of theall-solid lithium secondary battery during charging and discharging, and(C) determining a deterioration state of the all-solid lithium secondarybattery from the relationship between the discharging capacity and thecharging capacity.

In the determination method of the present disclosure, the deteriorationof the all-solid lithium secondary battery is deterioration that iscaused by growth of metallic lithium dendrites, and for example,metallic lithium dendrites grow and reach the metallic lithiumabsorption layer, and an irreversible capacity is generated, and therebythe discharging capacity is reduced.

In the determination method of the present disclosure, the all-solidbattery that has been determined to have deteriorated may be useddirectly or it may be replaced immediately or after it is additionallyused for a certain time or use conditions such as a charging anddischarging rate may be changed.

As shown in FIG. 3B, when the all-solid lithium secondary battery 10 cof the present disclosure is repeatedly charged and discharged, thedendrite 20 gradually grows from the side of the negative electrodeactive material layer 4 and reaches the metallic lithium absorptionlayer 7. The dendrite 20 that has reached the metallic lithiumabsorption layer 7 reacts with the metallic lithium reactive substancecontained in the metallic lithium absorption layer 7 to generate theelectron conductor 27 which is stable under battery charging anddischarging conditions.

Since the electron conductor 27 generated due to the reaction is stableunder battery charging and discharging conditions, no lithium ions aregenerated when the battery is discharged, and an irreversible capacityis generated. Then, when a dendrite grows and reaches the metalliclithium absorption layer, the discharging capacity of the all-solidlithium secondary battery is significantly lower than the chargingcapacity.

Therefore, in the all-solid lithium secondary battery of the presentdisclosure, when the charging and discharging capacities are measured,it is possible to detect the fact that metallic lithium dendrites havegrown and reached the metallic lithium absorption layer.

<Process (A)>

In the process (A), the all-solid lithium secondary battery of thepresent disclosure is charged and discharged. Charging and dischargingconditions are not particularly limited.

The charging and discharging conditions may be, for example, chargingand discharging conditions when the battery is used.

<Process (B)>

In the process (B), a charging capacity and a discharging capacity ofthe all-solid lithium secondary battery of the present disclosure aremeasured during charging and discharging.

Regarding the method of measuring a charging capacity and a dischargingcapacity, any method of measuring a charging capacity and a dischargingcapacity of a battery can be performed, and for example, charging anddischarging current amounts can be summed.

<Process (C)>

In the process (C), the deterioration state of the all-solid lithiumsecondary battery is determined from the relationship between thedischarging capacity and the charging capacity measured in the process(B).

The determination in the process (C) can be performed using any methodin which it is determined that the all-solid lithium secondary batteryhas deteriorated when a metallic lithium dendrite has grown and reachedthe metallic lithium absorption layer, and it is determined that theall-solid lithium secondary battery has not deteriorated when a metalliclithium dendrite has not grown and reached the metallic lithiumabsorption layer. Therefore, in the determination in the process (C), itis not always necessary to determine whether a metallic lithium dendritehas grown and reached the metallic lithium absorption layer.

It is preferable that the relationship between the discharging capacityand the charging capacity before a dendrite reaches the metallic lithiumabsorption layer and the relationship between the discharging capacityand the charging capacity when a dendrite grows and reaches the metalliclithium absorption layer can be identified each other in thedetermination in the process (C).

In the determination in the process (C), specifically, when thedifference between the discharging capacity and the charging capacity isequal to or larger than a threshold value, it may be determined that theall-solid lithium secondary battery has deteriorated. In addition, inanother determination method, when a proportion of the charging capacitywith respect to the discharging capacity is equal to or lower than athreshold value, it may be determined that the all-solid lithiumsecondary battery has deteriorated.

The threshold value can be determined as any value that can identify therelationship between the discharging capacity and the charging capacitybefore and after a dendrite reaches the metallic lithium absorptionlayer.

The threshold value may be determined so that, for example, when thesample of the all-solid lithium secondary battery of the presentdisclosure is charged and discharged, respective discharging capacitiesand charging capacities before a dendrite reaches the metallic lithiumabsorption layer and after a dendrite reaches the metallic lithiumabsorption layer are measured, the relationship between the dischargingcapacity and the charging capacity before a dendrite reaches themetallic lithium absorption layer and the relationship between thedischarging capacity and the charging capacity when a dendrite reachesthe metallic lithium absorption layer can be distinguished.

Example 1

Here, 50 mg of a halogen-containing Li—P—S solid electrolyte was weighedout and put into a ceramic die with an inner diameter of 11.28 mm (1cm²), and uniaxial molding was performed using a steel pin at a load of10 kN for 1 minute, and thereby a first layer made of thehalogen-containing Li—P—S solid electrolyte was molded. Here, the firstlayer was a solid electrolyte layer.

Next, 50 mg of a Li₁₀GP₂S₁₂ solid electrolyte as a metallic lithiumreactive substance was weighed out and put into the ceramic die from oneside of the first layer, and uniaxial molding was performed using asteel pin at a load of 5 kN for 1 minute, and thereby a second layermade of the Li₁₀GP₂S₁₂ solid electrolyte was molded. Here, the secondlayer is a metallic lithium absorption layer.

Next, 50 mg of a halogen-containing Li—P—S solid electrolyte was weighedout and put into a ceramic die from the side of the second layer, anduniaxial molding was performed using a steel pin at a load of 5 kN for 1minute, and thereby a third layer made of the halogen-containing Li—P—Ssolid electrolyte was molded. Here, the third layer was a solidelectrolyte layer.

Next, 40 mg of LiNi_(1/3)Co_(1/3)Mn_(1/3)O₂ as a positive electrodeactive material was weighed out and put into a ceramic die from the sideof the third layer, a copper foil with a thickness of 10 μm as anegative electrode current collector layer was put into a ceramic diefrom the side of the first layer, uniaxial molding was performed at aload of 60 kN for 3 minutes, a positive electrode active material layerwas molded on the side of the third layer, a negative electrode currentcollector layer was disposed on the side of the first layer, and therebya cell was completed.

Finally, the cell was restrained at a load of 250 kgf, and thereby anall-solid lithium secondary battery of Example 1 was prepared.

The prepared all-solid lithium secondary battery of Example 1 had thefirst layer (solid electrolyte layer), the second layer (metalliclithium absorption layer), and the third layer (solid electrolyte layer)between the negative electrode current collector layer and the positiveelectrode active material layer in this order from the side of thenegative electrode current collector layer.

Example 2

An all-solid lithium secondary battery of Example 2 was prepared in thesame manner as in Example 1 except that a Li—Si—P—S—Cl solid electrolytewas used in place of the Li₁₀GP₂S₁₂ solid electrolyte as the metalliclithium reactive substance. Here, the Li—Si—P—S—Cl solid electrolyte hadan LGPS type structure. The Li—Si—P—S—Cl solid electrolyte used in thefollowing Example 3 and Comparative Examples 3 and 4 also had the samestructure.

The prepared all-solid lithium secondary battery of Example 2 had thefirst layer (solid electrolyte layer), the second layer (metalliclithium absorption layer), and the third layer (solid electrolyte layer)between the negative electrode current collector layer and the positiveelectrode active material layer from the side of the negative electrodecurrent collector layer in this order.

Example 3

Here, 50 mg of a halogen-containing Li—P—S solid electrolyte was weighedout and put into a ceramic die with an inner diameter of 11.28 mm (1cm²), uniaxial molding was performed using a steel pin at a load of 10kN for 1 minute, and thereby a first layer made of thehalogen-containing Li—P—S solid electrolyte was molded. Here, the firstlayer was a solid electrolyte layer.

Next, 50 mg of a Li—Si—P—S—Cl solid electrolyte as the metallic lithiumreactive substance was weighed out and put into a ceramic die from oneside of the first layer, uniaxial molding was performed using a steelpin at a load of 5 kN for 1 minute, and thereby a second layer made ofthe Li—Si—P—S—Cl solid electrolyte was molded. Here, the second layerwas a metallic lithium absorption layer.

Next, 40 mg of LiNi_(1/3)Co_(1/3)Mn_(1/3)O₂ as a positive electrodeactive material was weighed out and put into a ceramic die from the sideof the second layer, a copper foil with a thickness of 10 μm was putinto a ceramic die from the side of the first layer, uniaxial moldingwas performed at a load of 60 kN for 3 minutes, a positive electrodeactive material layer was molded on the side of the second layer, anegative electrode current collector layer was disposed on the side ofthe first layer, and thereby a cell was completed.

Finally, the cell was restrained at a load of 250 kgf, and thereby anall-solid lithium secondary battery of Example 3 was prepared.

The prepared all-solid lithium secondary battery of Example 3 had thefirst layer (solid electrolyte layer) and the second layer (metalliclithium absorption layer) between the negative electrode currentcollector layer and the positive electrode active material layer fromthe side of the negative electrode current collector layer in thisorder.

When the all-solid lithium secondary battery is being charged in thecharging and discharging test 1 described below for the all-solidlithium secondary battery of Examples 1 to 3, a metal lithium isdeposited on a surface of the negative electrode current collector,which is in contact with the first layer, whereby a negative electrodeactive material layer (Li layer) is self-formed.

Comparative Example 1

Here, 150 mg of a halogen-containing Li—P—S solid electrolyte wasweighed out and put into a ceramic die with an inner diameter of 11.28mm (1 cm²), uniaxial molding was performed using a steel pin at a loadof 10 kN for 1 minute, and thereby a first layer made of thehalogen-containing Li—P—S solid electrolyte was molded. Here, the firstlayer was a solid electrolyte layer.

Next, 40 mg of LiNi_(1/3)Co_(1/3)Mn_(1/3)O₂ as a positive electrodeactive material was weighed out and put into a ceramic die from one sideof the first layer, a copper foil with a thickness of 10 μm was put intoa ceramic die from the other side of the first layer, uniaxial moldingwas performed at a load of 60 kN for 3 minutes, a positive electrodeactive material layer was molded on one side of the first layer, anegative electrode current collector layer was disposed on the otherside, and thereby a cell was molded.

Finally, the cell was restrained at a load of 250 kgf, and thereby anall-solid lithium secondary battery of Comparative Example 1 wasprepared.

The prepared all-solid lithium secondary battery of Comparative Example1 had only the first layer (solid electrolyte layer) between thenegative electrode current collector layer and the positive electrodeactive material layer.

Comparative Example 2

An all-solid lithium secondary battery of Comparative Example 2 wasprepared in the same manner as in Example 1 except that a Li₇P₃S₁₁ solidelectrolyte as a solid electrolyte was used in place of the Li₁₀GP₂S₁₂solid electrolyte as the metallic lithium reactive substance. Here, theLi₇P₃S₁₁ solid electrolyte was a solid electrolyte having significantlylow reactivity with metallic lithium.

The prepared all-solid lithium secondary battery of Comparative Example2 had the first layer (solid electrolyte layer), the second layer (layermade of a Li₇P₃S₁₁ solid electrolyte), and the third layer (solidelectrolyte layer) between the negative electrode current collectorlayer and the positive electrode active material layer from the side ofthe negative electrode current collector layer in this order.

Comparative Example 3

An all-solid lithium secondary battery of Comparative Example 3 wasprepared in the same manner as in Comparative Example 1 except that aLi—Si—P—S—Cl solid electrolyte as a metallic lithium reactive substancewas used in place of the halogen-containing Li—P—S solid electrolytehaving significantly low reactivity with metallic lithium.

The prepared all-solid lithium secondary battery of Comparative Example3 had only the first layer (metallic lithium absorption layer) betweenthe negative electrode current collector layer and the positiveelectrode active material layer.

Comparative Example 4

An all-solid lithium secondary battery of Comparative Example 4 wasprepared in the same manner as in Example 3 except that a laminationorder of the solid electrolyte layer and the metallic lithium absorptionlayer was changed.

Here, the prepared all-solid lithium secondary battery of ComparativeExample 4 had the first layer (metallic lithium absorption layer) andthe second layer (solid electrolyte layer) between the negativeelectrode current collector layer and the positive electrode activematerial layer from the side of the negative electrode current collectorlayer in this order.

«Measurement of Charging and Discharging Capacities» <Charging andDischarging Test 1>

The all-solid lithium secondary batteries of Examples 1 to 3 andComparative Examples 1 to 4 were charged and discharged under conditionsof a lower limit voltage of 3.0 V, an upper limit voltage of 4.37 V, acharging and discharging rate of 0.1 C, and a current density of 456μA/cm², that is, under conditions of a low current density, and therebyit was checked whether these batteries operated.

The all-solid lithium secondary batteries of Examples 1 to 3, andComparative Examples 1 and 2 operated as batteries, but ComparativeExamples 3 and 4 did not operate as batteries.

<Charging and Discharging Test 2>

The all-solid lithium secondary batteries of Examples 1 to 3 andComparative Examples 1 and 2 of which functions as the battery wereconfirmed were charged and discharged under conditions of a lower limitvoltage of 3.0 V, an upper limit voltage of 4.37 V, a charging anddischarging rate of 2.0 C, and a current density of 9.12 mA/cm², thatis, conditions of a high current density, and thereby the chargingcapacity and the discharging capacity of the all-solid lithium secondarybatteries were measured.

RESULTS AND CONCLUSIONS

Table 1 shows the configurations of the all-solid lithium secondarybatteries and results of the above two charging and discharging tests.In addition, FIGS. 4 to 10 show graphs showing charging and dischargingcapacities of the all-solid lithium secondary batteries when thecharging and discharging tests 1 and 2 were performed. Here, in FIGS. 4to 10, solid line and dotted line graphs show measurement results ofcharging and discharging capacities of the all-solid lithium secondarybatteries when charging and discharging were performed according to thecharging and discharging tests 1 and 2.

TABLE 1 Measurement results Configurations of layers between negativeelectrode Charging and Charging and current collector layer and positiveelectrode active discharging test discharging test material layer 1 (lowcurrent 2 (high current Examples First layer Second layer Third layerdensity) density) Example 1 Solid Metallic Solid Operated No shortelectrolyte lithium electrolyte circuiting layer absorption layer Lowlayer discharging capacity Example 2 Solid Metallic Solid Operated Noshort electrolyte lithium electrolyte circuiting layer absorption layerLow layer discharging capacity Example 3 Solid Metallic — Operated Noshort electrolyte lithium circuiting layer absorption Low layerdischarging capacity Comparative Solid — — Operated Short circuitingExample 1 electrolyte occurred layer Comparative Solid Solid SolidOperated Short circuiting Example 2 electrolyte electrolyte electrolyteoccurred layer layer layer Comparative Metallic — — Not operated —Example 3 lithium absorption layer Comparative Metallic Solid — Notoperated — Example 4 lithium electrolyte absorption layer layer

As shown in the solid line graphs in FIGS. 4 to 8 and Table 1, in theall-solid lithium secondary batteries of Examples 1 to 3 and ComparativeExamples 1 and 2, the operation as the battery was confirmed duringcharging and discharging at a low current density.

Comparing the all-solid lithium secondary batteries of Examples 1 and 2and Comparative Examples 1 and 2, and the all-solid lithium secondarybattery of Example 3, the all-solid lithium secondary batteries ofExamples 1 and 2 and Comparative Examples 1 and 2 had a dischargingcapacity of 4 mAh/g or more, the all-solid lithium secondary battery ofExample 3 had a discharging capacity of about 3 mAh/g, and the all-solidlithium secondary battery of Example 3 had a lower discharging capacitythan the all-solid lithium secondary batteries of Examples 1 and 2 andComparative Examples 1 and 2.

However, in all of the all-solid lithium secondary batteries of Examples1 to 3, and Comparative Examples 1 and 2, the operation as the batterywas not confirmed.

On the other hand, as shown in Table 1 and FIGS. 9 and 10, in theall-solid lithium secondary batteries of Comparative Examples 3 and 4,no discharging occurred, and the operation as the battery was notconfirmed.

The reason why the all-solid lithium secondary batteries of ComparativeExamples 3 and 4 did not operate is speculated to be as follows. TheLi—Si—P—S—Cl solid electrolyte as the metallic lithium reactivesubstance contained in the first layer in contact with the negativeelectrode current collector layer reacted with metallic lithiumprecipitated on the negative electrode current collector layer duringcharging to generate a stable electron conductor. Therefore, lithiumions could not move from the side of the negative electrode currentcollector layer to the side of the positive electrode active materiallayer during discharging.

(Charging and Discharging Test 2: Charging and Discharging Test at aHigh Current Density)

As shown in the dotted line graphs in FIGS. 7 to 8 and Table 1, in theall-solid lithium secondary batteries of Comparative Examples 1 and 2,the voltage did not increase during charging. This indicates thatinternal short circuiting occurred in the all-solid lithium secondarybatteries of Comparative Examples 1 and 2 due to charging anddischarging at a high current density.

On the other hand, as shown in the dotted line graphs in FIGS. 4 to 6and Table 1, in the all-solid lithium secondary batteries of Examples 1to 3, the voltage increased to the upper limit voltage during charging.

The results show that no internal short circuiting occurred in theall-solid lithium secondary batteries of Examples 1 to 3 when chargingand discharging were performed at a high current density.

In addition, as shown in the dotted lines in FIGS. 4 to 6 and Table 1,in the all-solid lithium secondary batteries of Examples 1 to 3, thedischarging capacity with respect to the charging capacity wassignificantly low during charging and discharging at a high currentdensity. When the all-solid lithium secondary batteries of Examples 1 to3 were disassembled after charging and discharging were performed at ahigh current density and respective layers were observed, the metalliclithium absorption layer was discolored black.

Based on the results, it was thought that, after the metallic lithiumdendrite reached the metallic lithium absorption layer, metallic lithiumreacted with the Li₁₀GP₂S₁₂ solid electrolyte and the Li—Si—P—S—Cl solidelectrolyte in the metallic lithium absorption layer to generate astable electron conductor, and thereby an irreversible capacity wasgenerated in these all-solid lithium secondary batteries.

What is claimed is:
 1. An all-solid lithium secondary battery,comprising: a positive electrode active material layer; a metalliclithium absorption layer containing a metallic lithium reactivesubstance that reacts with metallic lithium to generate an electronconductor which is stable under charging and discharging conditions ofthe all-solid lithium secondary battery; a first solid electrolytelayer; and a negative electrode active material layer that is in contactwith the first solid electrolyte layer, wherein the positive electrodeactive material layer, the metallic lithium absorption layer, the firstsolid electrolyte layer, and the negative electrode active materiallayer are disposed in this order.
 2. The all-solid lithium secondarybattery according to claim 1, further comprising a second solidelectrolyte layer between the positive electrode active material layerand the metallic lithium absorption layer.
 3. The all-solid lithiumsecondary battery according to claim 1, wherein the metallic lithiumreactive substance has lithium ion conductivity.
 4. The all-solidlithium secondary battery according to claim 1, wherein the metalliclithium reactive substance is a solid electrolyte containing Li, P, S,and M, and wherein M is Ge, Si, Sn, or a combination thereof.
 5. Theall-solid lithium secondary battery according to claim 4, wherein themetallic lithium reactive substance is a Li_(3.25)Ge_(0.25)P_(0.75)S₄,Li₁₀GeP₂S₁₂, Li₁₀SnP₂S₁₂, Li₁₁Si₂PS₁₂, or Li₄GeS₄—Li₃PS₄ glass ceramic,a Li—Si—P—S—Cl solid electrolyte having an LGPS type structure, or acombination thereof.
 6. The all-solid lithium secondary batteryaccording to claim 1, wherein the negative electrode active materiallayer contains the metallic lithium.
 7. A method of determining adeterioration state of an all-solid lithium secondary battery,comprising: a first process of charging and discharging the all-solidlithium secondary battery according to claim 1; a second process ofmeasuring a charging capacity and a discharging capacity of theall-solid lithium secondary battery during the charging and discharging;and a third process of determining the deterioration state of theall-solid lithium secondary battery from a relationship between thedischarging capacity and the charging capacity.
 8. The method accordingto claim 7, wherein, in the third process, when a difference between thedischarging capacity and the charging capacity is equal to or greaterthan a first threshold value or when a proportion of the chargingcapacity with respect to the discharging capacity is equal to or lowerthan a second threshold value, it is determined that the all-solidlithium secondary battery has deteriorated.